1. Technical Field of the Invention
The embodiments of the invention relate to communication devices and more particularly to an offset cancellation scheme for a mixer.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Communication systems typically operate in accordance with one or more communication standards. For instance, wired communication systems may operate according to one or more versions of the Ethernet standard, the System Packet Interface (SPI) standard, or various other standards. Wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). Typically, the transceiver includes a data modulation stage and an RF stage. The data modulation stage (baseband process) converts between data and baseband signals in accordance with the particular wireless communication standard. The RF stage (transmitter section and receiver section) converts between baseband signals and RF signals. The RF stage may be a direct conversion transceiver that converts directly between baseband and RF or may include one or more intermediate frequency stages.
In some signal processing techniques employed for wireless communication, a modulation (and/or demodulation) stage processes the in-phase (I) component and the quadrature (Q) component of the complex modulation envelope separately. For example, since the I and Q components have a phase difference, an up-conversion module in a signal transmitter path may utilize one mixer to mix the signal with an in-phase local oscillator signal to generate the I component and utilize a second mixer to mix the signal with a phase-shifted local oscillator signal to generate the Q component.
A common type of mixer utilized in communication is a Gilbert cell mixer. A typical mixer stage, including a Gilbert cell mixer, is driven by a transconductance stage, in which input signal voltage is converted to signal current to drive the mixer. The transconductance stage may have gain control circuitry to adjust the gain of the drive current to the mixer. However, in a typical operation a transconductance stage generally generates some amount of offset, since not all of the transistors in the transconductance stage are matched. This offset may be represented as a small current that adds or subtracts from the signal current and when coupled to the mixer the offset current may cause a local oscillator (LO) feedthrough at the mixer. LO feedthrough is a problem if it exceeds a leakage amount specified by a communication standard or if the feedthrough is sufficient to cause the receiver to not receive the signal properly. The offset may be controlled by determining the amount of the LO feedthrough (LOFT) and introducing a cancellation current at the appropriate location in the transconductance stage. The cancellation current may then cancel the effect of the offset to remove the LOFT.
However, since offsets are the result of unbalanced or un-matched circuit characteristics, the offset values in the I transconductance stage may differ from the offset in the Q transconductance stage. In that event, introducing appropriate LOFT cancellation currents in each of the I and Q stages may remove the offsets, but the difference in the cancellation currents introduced into the I and Q circuitry causes an I and Q imbalance, since the total mixer currents between the I and Q mixers are not equal. Thus, even though offsets are dealt with, an IQ imbalance may result from the offset correction.
Accordingly, it would be advantageous to implement an offset cancellation scheme without generating an I/Q imbalance in the I and Q mixers.